A. Field of the Invention
The present invention relates to a thermal prim head. More particularly, the present invention relates to a thermal print head in which a cooling board is divided into two regions.
B. Description of the Prior Art
Thermal recording is a technique by which characters or graphics are recorded onto thermal paper. In thermal recording, only heated portions of white thermal paper are darkened into black portions. By regulating the portions of the thermal paper that are heated, the desired characters or graphics can be placed on the thermal paper. A thermal printer is a machine to which the above-mentioned technique of thermal recording is applied. A thermal printer uses a thermal print head on which heating elements for converting electrical energy into heat energy are formed in a line. Each heating element in the line forms a dot on the paper when it is heated.
When a user prints characters or graphics using the thermal printer, the heating elements are selectively heated to generate heat based on data inputted as an electrical signal while the thermal print head contacts a medium such as the thermal paper. The generated heat is applied to the thermal paper to record the characters or graphics as a series of dots. The image is formed by sequentially applying lines of data, i.e., heated dots, to the thermal paper.
A conventional thermal print head will be described in detail below with reference to the accompanying drawings.
FIG. 1 is a sectional view of the conventional thermal print head, and FIG. 2 is a perspective view illustrating a portion corresponding to a reference number, 10. Referring to FIG. 1, the conventional thermal print head comprises a resistance substrate 1, a plurality of heating elements 2, a driving integrated circuit 21, and a driving substrate 30. The resistance substrate 1 is made of ceramics or the like and has the plurality of heating elements 2 formed on a front surface. Each of the plurality of heating elements 2 generates heat when current flows through it. The driving integrated circuit 21 is formed on the driving substrate 30 and separately drives the resistance substrate 1 and the heating elements 2.
Referring to FIG. 2, the heating elements 2 are linearly formed in a predetermined direction like dots. They are driven and heated selectively and generate heat to the thermal paper. A cooling board 3 is adhered to a rear surface of the resistance substrate 1 by an adhesive 4. The cooling board 3 is made of metal with high thermal conductivity to effectively dissipate heat generated by the heating elements 2. A part of the cooling board 3 is connected to a part of the driving substrate 30 by the adhesive 4, thereby supporting the driving substrate 30. A connector 40 is then supports the remainder of the driving substrate 30.
A protector 22 protects the driving integrated circuit 21 by covering the driving integrated circuit 21. A cover 20 further protects the driving integrated circuit by covering over the protector 22. The cover 20 is attached to the driving substrate 30 by a screw 50.
In operation, when the thermal print head is used at room temperature, the surface temperature of the resistance substrate 1 can reach 200.degree. C. because of the heat generated from the heating elements 2. Accordingly the printer must dissipate any excess heat that does not contribute to printing or it will adversely influence printing jobs performed by the printer. When heat dissipation has not been performed efficiently, the undissipated heat causes uneven contrast in the current printing job.
To solve this problem, the cooling board 3, which is made of a metal with a high thermal conductivity, is attached to the rear surface of the resistance substrate 1 to enhance dissipation of excess heat generated by the heating elements 2.
Generally, double-sided tape is used as the adhesive 4 to attach the cooling board 3 to the resistance substrate 1. Double-sided tape is used because the resistance substrate 1 and the cooling board 3 differ significantly in their coefficients of thermal expansion. If the resistance substrate 1 and the cooling board 3 were fixed directly, they would be warped by the heat during printing in the same way that a bimetal is warped by changes in its ambient temperature. The use of double sided tape reduces the stress caused by the difference of the coefficients of thermal expansion of the resistance substrate 1 and the cooling board 3 and thereby prevents warping of the resistance substrate 1. In addition, using double-sided tape also simplifies production of the printer by making it easier to join the cooling board 3 to the resistance substrate 1.
Double-sided tape is not without its problems, however. When the resistance substrate 1 and the cooling board 3 are joined by double-sided tape, heat generated by the resistance substrate 1 is not sufficiently dissipated into the cooling board 3. This occurs because the thermal conductivity of the double-sided tape is generally small, for example, less than 0.5.times.10.sup.-3 cal/cm-sec-.degree.C. The incomplete heat dissipation caused by using the double-sided tape can result in uneven contrast and smearing of the printed image because too much heat may remain in the resistance substrate 1. Accordingly, the double-sided tape cannot be used in high speed printers, color printers, high speed label printers, and the like, which require greater heat dissipation.
To overcome the above-mentioned disadvantage, a cooling compound on the principal plane of the resistance substrate 1 can be used in place of the adhesive 4. Unfortunately, the thickness of the cooling compound can differ locally along the resistance substrate 1 because the manufacturing process results in areas having a difference in height on the order of tens to hundreds of microns. The uneven thickness in turn causes uneven cooling and therefore uneven contrast in the printed image.
A technique has been proposed, however, to solve the above-mentioned disadvantages. This technique will be explained below with reference to FIG. 3.
As shown in FIG. 3, a cooling material or a cooling compound 6 is used in addition to the adhesive 4. The cooling compound is inserted between the cooling board 3 and the rear surface of the resistance substrate 1, in an area corresponding to the area on the front surface of the resistance substrate 1 in which the heating elements 2 are formed. Two long grooves 5 are formed in the cooling board 3 in the same direction in which the heating elements 2 are arranged. The adhesive 4 for adhering the resistance substrate 1 to the cooling board 3 is positioned at both sides of the cooling compound 6, bordering on the grooves 5.
The cooling compound 6 is preferably a mixture of silicon oil and fine particles of aluminum oxide or zinc oxide of a size of 1 .mu.m or less. This results in a viscous cooling compound 6 with a thermal conductivity preferably in the range of 1.5.times.10.sup.-3 to 3.0.times.10.sup.-3 cal/cm-sec-.degree.C. Thus, the cooling compound 6 has a thermal conductivity 3 to 6 times that of the adhesive 4 of the foregoing prior art.
After applying the cooling compound 6 on the cooling board 3 between the long grooves 5, the cooling compound 6 is compressed and evenly spread when the resistance substrate 1 is pressed on the cooling board 3. The gaps provided by the long grooves 5 allow the cooling compound 6 space to spill over, assuring that its thickness will remain even.
However, the prior art as illustrated in FIG. 3 has a disadvantage that the entire cooling board 3 must be removed from the resistance substrate 1 and the driving substrate 30 in order to correct any problems. These problems include when an air bubble is formed in the adhesive 4 or the cooling compound 6, when the resistance substrate 1 is adhered to the cooling board 3 but the adhesive 4 or the cooling compound is not evenly deposited, or when the cooling compound 6 must be again deposited again after the resistance substrate and the cooling board have already been connected.